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The -related mafic dyke complex of the Rohkunborri , Indre Troms, northern Norwegian Caledonides

LARS KRISTIAN STØLEN

Stølen, L. K.: The rift-related mafic dyke complex of the Rohkunborri Nappe, Indre Troms, northern Norwegian Caledonides. Norsk Geologisk Tidsskrift, Vol. 74, pp. 35-47. Oslo 1994. ISSN 0029-196X.

Well-preserved dyke complexes are found within the Seve Nappe Complex, Upper , in the easternmost part of Indre Troms, Norway. They form a separate thrust sheet, the Rohkunborri Nappe, and are best exposed in two low-strain units in the Rohkunborri and Njunis areas. In these areas the dolerite swarm intrudes a succession of carbonate rocks and subordinate sandstones, siltstones and quartzites. The sedimentary rocks, the Nj unis Group, are supposed to represent the outermost part of the clastic wedge deposited during Neoproterozoic-earliest Palaeozoic time on the thinned outermost part of the Baltoscandian margin. A complex structural evolution can be recognized within the dyke complexes involving early block rotation d.uring intrusion, rotation of both dykes and bedding due to simple deformation during thrusting and subsequent imbrication of the dyke complex. The Indre Troms dykes are part of the Neoproterozoic- Early Palaeozoic rifted Baltoscandian margin and represent the transition zone between the continent Baltica and the . The Rohkunborri Nappe can be correlated with the Vaivvancohkka Nappe and the Sarektjåkkå Nappe of the Seve Nappe Complex in the northemmost Swedish Caledonides.

Lars Kristian Stølen, Department of Mineralogy and Petrology, Institute of Geo/ogy, University of Lund, Solvegatan 13, S-223 62 Lund, Sweden.

Introduction the latest Proterozoic to Early Cambrian of baltica (Gee 1975; Andreasson 1987). Mafic dyke swarms are known from several parts of the The tectonic units of the Koli Nappe Complex that Scandinavian Caledonides. They occur in the overlie the Seve units locally are composed of a variety of and lower thrust sheets in the northernmost parts of the . derived from outboard of Baltica (Stephens & mountain belt; further south, they occur only in the Gee 1989). At least two episodes of mafic igneous activ­ . According to a terminology introduced by ity related to the development of and are Kulling ( 1972) and Gee & Zachrisson (1979), the al­ spreading are recorded within the Koli Nappe Complex. lochthons can be divided into four major tectonostrati­ One is of Early Ordovician age and represented by the graphic units: Lower, Middle, Upper and Uppermost. The Lower Allochthon is dominated by Vendian and Karmøy and Leka complexes (Furnes et al. Lower Palaeozoic successions, the Middle Allochthon by 1988 ; Pedersen et al. 1988 ; Pedersen & Hertogen 1990) ; thick Vendian and Upper Proterozoic strata, together the other is Late Ordovician to earliest Silurian in age with sheets of Precambrian crystalline rocks. The succes­ and represented by the Solund/Stavfjorden ophio1ite sions of the Lower Allochthon are readily correlated with (Furnes et al. 1980; Dunning and Pedersen 1988). Mafic those of the Baltoscandian Platform Autochthon; those complexes imruded during rifting of volcanic arcs and in the Middle Allochthon are supposed to have been fo rmation of marginal basins in the northern Scandina­ derived fr om the Baltoscandian miogeocline. vian Caledonides, e.g. the Sulitjelma lgneous Complex The uppermost tectonic units of the Middle Al­ (Pedersen et al. 1991) and the Råna Massif (Tucker et al. lochthon (Sarv Nappe) were thrust fr om the outer part 1990), have a similar age to the Solund/Stavfjorden of the Baltoscandian miogeocline; they are extensively ophiolite. intruded by mafic dyke swarms (Stromberg 1969) related As part of a more comprehensive study of the relation­ to Late Proterozoic rifting (Gee 1975; Solyom et al. ships between inboard and outboard terranes in northern 1979; Claesson & Roddick 1983), which extended more parts of the Scandinavian Caledonides, this study deals than l 000 km along the Baltoscandian margin (Andreas­ with mafic dyke swarms occurring in the easternmost son 1987). parts of Indre Troms, and fo cuses on their made of The lower part of the overlying Upper Allochthon, occurrence and tectonostratigraphic position. Previous composed of the Seve Nappe Complex, is an imbricate work, e.g. Gee et al. ( 1985a, b) , has inferred a late stack of lithotectonic units (Andreasson 1986) character­ Proterozoic age for the dykes and has correlated these ized by tholeiitic metabasites (locally as dyke swarms) units with those occurring in the Seve Nappe Complex in and psammitic schists. lts complexes are also inferred to northern Sweden. The evidence presented here supports be indigenous, and derived from the outermost part of this hypothesis. 36 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFT 74 (1994)

The Baltoscandian margin nized in Sweden as far north as central Norrbotten (Fig. I). The more complexly deformed a�d highly �etamor­ In the central part of the Scandinavian Caledonides (Fig. phosed Seve Nappe Complex, overlymg the Mtddle Al­ I) evidence from the Lower and Middle Allochthons lochthon in Sweden, contains a similar, but somewhat provides constraints on the evolution of the Bal�oscan­ more complex protolith, including both volcano-sedi­ dian margin. Stratigraphic correlations (Kumpulamen & mentary Baltoscandian miogeoclinical succes�ions (A�­ Nystuen 1985) of Late Proterozoic to Lower Cambrian dn!asson 1987) and elements of the Proterozotc cratomc sedimentary rocks within these thrust sheets have estab­ margin. lished three informal units: a thick lower mainly fluviatile The lower part of the Seve Nappe Complex in Nor­ unit, a middle glacial unit (Varangian) and an upper rbotten, the Vaimok Lens (Fig. 2), contains a dolerite-in­ fluviatile and shallow marine unit of Late Vendian to truded metasedimentary sequence, with similarities to the Early Cambrian age. In northem areas, the upper unit underlying Sarv Nappe, but metamorphosed to ec��git: dominates the autochthon and the middle unit is also grade. These units are overthrust by the Sarektjakka locally preserved; further south all three units are gener­ Nappe (Svenningsen 1987), which is dominated by Ven­ ally allochthonous and the autochthonous strata · are dian mafic dykes (8 0%), occurring as sheeted complexes, mainly Mid-Cambrian to Ordovician. The Vendian and with screens of quartzites, meta-arkoses and subordinate Late Proterozoic successions can be readily correlated carbonates, showing similarities with the Tossåsfjellet with sequences within the Middle Allochthon at !east as Group of the Sarv (Kumpulainen 1980). far north as the Akkajaure area (Kumpulainen & Further north, in the Kebnekaise area, the nearly Nystuen 1985). l 00 % dyke-dominated Kebne dyke complex (Andreas­ Stratigraphical and sedimentological studies (Kumpu­ son & Gee 1989) can be correlated with the Sarektjåkkå lainen 1980; Kumpulainen & Nystuen 1985) indicate that Nappe. Near the Norwegian border in the Tometrask the pre-Vendian successions were deposited in rift-related area, Kathol ( 1989) mapped a sequence of dyke-intruded basins along the Baltoscandian margin. The extensive sandstones with cross- and graded-bedding in the Vaiv­ Late Proterozoic tholeiitic dyke swarms of the Sarv vancohkka Nappe, overlying augen with eclogi­ Nappe suggest several tens of kilometres of passive mar­ tized metabasite lenses. He included all these units in the gin extension. These dyke complexes have been recog- Seve Nappe Complex. The high-grade Seve augen gneisses, together with units from the underlying Middle Allochthon, can be followed northwards from Tome­ trask ' via the area of Indre Troms, treated in more detail here, to Treriksrøysa (Stølen 1989), where the Seve units pro bably wedge out (e.f. Zachrisson 1986). ' -­ _ _ , In northem Norway, mafic dyke swarms occur in units ' correlated (Gee et al. l985a, b) with the upper part of , , Middle Allochthon and the overlying Vaddas (Koli) Map area fig. 3 Nappe. In the upper Middle Allochthon, the dyke-in­ truded units are closely comparable with those in the Sarv Nappe (and Seve in Norrbotten) and have been correlated with them (e.g. the Corrovarre Nappe of 1100 km l Zwaan & van Roermund 1990). In the Vaddas Nappe the mafic rocks occur both as dykes and as in a LEGEND : � Late Ordovician succession. Thus, in this part of the N Uppermost Allochthon mountain belt, dyke complexes that were injected in the Vendian and in the Ordovician are juxtaposed. D UpperAllochthon : \ b:::l KOii Nappe Complex The Corrovarre dykes have been dated to c. 580 Ma (Zwaan & van Roermund 1990) and correlated with • Upper Allochthon : Seve Nappe Complex those in the Seiland Igneous Province. The latter are thought to have formed during thinning and rifting of R Lower and Middle l::1::jAllochthon the Baltoscandian shield prior to Finnmarkian deforma­ tion (Roberts 1990; Zwaan & van Roermund 1990). • Autochthonous The Vaddas Nappe contains fossiliferous rocks of Late Ordovician or Early Silurian age (Binns & Gayer 1980) r-::::1 Para·autochthonous t;;:Jcrystal line basement and Late Ordovician basalts. The lower part of the rocks in windows Vaddas Nappe is composed of interlayered marb1es and schist overlain by a quartzite-dominated unit (Padget Fig. 1. Simplifiedtectonostratigraphic map of the northem part of the Scandina­ 1955). These units are overlain by a sequence of fossili­ vian Caledonides. Place names related to profiles in Fig. 2: I= Indre Troms, J = Jiimtland, K = Kebnekaise, N = northem Troms, T = Tometriisk-Ofoten­ ferous marble and mafic volcanics, locally pillow . A southem Troms, S = Sarek-Padjelanta (Norrbotten) and V=Viisterbotten. monomictic conglomerate with quartzite clasts marks the NORSK GEOLOGISK TIDSSKRIFT 74 (1994) Mafic dyke complex, Rohkunborri Nappe, Troms 37

Jiimttand Viisterbotten Sarek- Kebnekaise Tornetriisk • Indre Troms Northem Troms Padjelanta Ofoten- S Troms

Geeet.al.198S Sccphens et.al. 198S Zachriuon & Sccphens1984 AndteassooGee & 1989 Katho11989 Thispoper AndresenZwun & inpress SjOstrom1983 Svcm:ing�m 1987 Page1993 Gustavson1974 Zwun&. vm Roennund1990 Uppermost Bj0ddw>di98S Kulling1964 Allochthon Barker1986

� .. li-..!! zo. �E �8 � "' (OT) � a: (IT) w .... Q Kåfjord Nappe � � l:; � o i;..E Q. (1)8

Lower Allochthon Autochthon

e Mafic intrusives in outboard terranes eJ Rin-related maficdyke swarms formed �ring opening of lapatus

Fig. 2. Tectonostratigraphic correlations of mafic dyke complexes within the Middle and Upper Allochthon between Jiimtland and northem Troms. Age data: l. Pedersen et al. (1991), 2. Svenningsen in press (a), 3. Tucker et al. (1990), 4. Stølen unpublished, 5. Zwaan & Roermund (1990), 6. Binns & Gayer (1980).

base of a superimposed series of schists and psammites, igneous textures and contammg lenses of marble and representing distal turbidites (Andresen 1988) ; these schist (Kalsbeek & Olesen 1967; Mortensen 1972; Gus­ rocks are overthrust by greenschists and . A tavson 1974). However, primary features of the interior large mafic complex, the Vaddas gabbro (Lindahl 1974), parts of these massifs were not reported, and intrudes lithologies in the upper part of the Vaddas these provide the main topic of this paper. Nappe. These units continue southwards towards the Regional compilations (Gee et al. 1985a, b; Zachrisson Treriksrøysa area ( Figs. l and 2). No ne of the abo ve 1986) correlated all the mafic dyke complexes of the mentioned units can be directly mapped into the Alte­ Indre Troms area with the Seve Nappe Complex. Some vatn-Dividalen area (Fig. 3) where the dyke complexes, workers (Krill et al. 1987, and Stephens & Gee 1989) treated further below, are located. correlated the southemmost one (Rohkunborri) with the Seve Nappe Complex and the rest (Kistefjell, Njunis and Jerta) north of this as part of the Koli Nappe Complex. The tectonic units exposed in the Altevatn-Dividalen The Altevatn-Dividalen area area are described briefty below, prior to a more detailed The Indre Troms area is dominated by ftat-lying thrust treatment of the dyke complexes. sheets underlain by the Late Vendian to Mid-Cambrian autochthonous sediments of the Dividal Group, which were deposited unconformably on the mostly Late Middle Allochthon Archean granitoid gneisses of the northem part of Baltic Shield (Føyn 1967). The lowest tectonostratigraphic unit in the Altevatn-Di­ Kalsbeek & Olesen ( 1967) divided the allochthonous vidalen area overlying the autochthonous Dividal Group units in the Altevatn-Dividalen area into three units. is the Målselv Nappe of the Middle Allochthon, consist­ Earlier workers had shown that the so-called 'Seve am­ ing of various phyllites, quartzites, dolomites, mylonitic phibolites' in the eastemmost part of Indre Troms con­ 'basement' rocks and tectonized clastic sedimentary sisted of foliated amphibolites, commonly with relict rocks, the so-called 'hard-schists' of Pettersen ( 1878). 38 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFT 74 (1994)

srøysa. It is correlated with the Storg1aciåren LEG END �/ (Andreasson & Gee 1989; Nilsson 1992) in the Keb­ N_;: al..---- WZJ KOii NappeComplex undifferenti ted �- � nekaise-Abiskofjållen area. \ Sweden Finland � Rohkunborri Nappe \ a The mafic dyke complexes described below are situated �c;:;;;:;;:.j Seve N ppe Co plex above the augen gneiss, separated from it by some tens (-_-_-_-_-_-_-_J Middle Allochthonm : Målselv Nappe mll'Jil Autochthonousa sedi ents: Dividal G oup of metres of . This thrust sheet, the Rohkun­ i n s men r borri Nappe (Stølen 1989), is interpreted to compose the m r m

\ . - · uppermost part of the Seve Nappe Complex in the Section . ::;�;::;:;:;:;:;:;:;:;:{��m::��Wlt�:J c:::::::J Preca b a ea e�;�� :�': : Altevatn-Dividalen area.

Upper Allochthon: Koli Nappe Complex The dyke complex is overthrust by garnet mica schists, locally with staurolite and kyanite, marbles, garben­ schists and subordinate amphibolites of the Koli Nappe Complex. A more detailed description of the Koli Nappe Complex is found elsewhere (Stølen, unpublished). Pl.ce nameerelerred to in text

A : Anjavassdalen B : Bangfjellat Tectonothermal history N G : Giimmariehppi Gv : Gamlnajajavri H : Høgskardet Four phases of deformation can be recognized within the J :Jerta i K: Kistefjell various thrust sheets of the Altevatn-Di vidalen area. M : Måskanvarri N :Njunis Evidence for Dl has been observed only within the Seve 15km R : RoMwnborri T:ljuo1jmer Nappes, as shown by early isoclinical folds and inclusion trails in garnet prophyroblasts. The D2 deformation is Fig. 3. Geological map of the Altevatn-Dividalen area (Indre Troms). Modified from Gustavson ( 1974) and Gustavson & Skålvoll ( 1977). related to development of the dominant schistosity in all the thrust sheets. 02, although similar in appearance in all thrust sheets, probably developed successively during These units dominate the low-lying areas around Alte­ emplacement of the nappes. F2 folds are recumbent vatn and in Anjavassdalen. A good marker horizon isoclinal with NE-SW trending axes, and S2 as axial within the Målselv Nappe is the characteristic yellow planes. Abisko dolomite (Kulling 1964), which can be recog­ Early 02 growth of garnet and kyanite within the Seve nized throughout the area. derived Nappe Complex, and only of garnet within the Koli from basement slices makes up the lowest part of the Nappes, took place at the peak of . S-C Målselv Nappe in the Jerta area. This rock unit, consist­ mylonites and extensional cleavages are de­ ing of coarse mylonitic fractured granitoids, could be veloped in thrust zones. The regional Caledonian trans­ part of the Lower Allochthon, correlatable with similar port direction in the area is to the ESE. units in the Rautas Nappe (Kulling 1964) in the Torne­ The third (03) and fourth (D4) deformation phases tråsk area. However, the contact is poorly exposed and post-date the thrusting; they fold thrust contacts in tight for the purpose of this paper it is regarded as an internat to open regional folds on N-S and NW-SE axes, re­ unit at the base of the Målselv Nappe. spectively. F3 folds are inclined, close to tight. Locally the S3 crenulation fabric is penetrative. 04 deformation is dominated by mesoscopic open to box shaped, asym­ Upper A/lochthon: Seve Nappe Comp/ex metric folding with towards SE. Kink banding The overlying Seve Nappe Complex is divided into three and crenulation cleavages are commonly developed, and sub-units in the Altevatn-Dividalen area. The lowermost locally the crenulation is penetrative. part is composed of partly phyllonitic pelitic to psam­ mitic gneisses with quartz segregations and subordinate quartz ± garnet mica schists. These rocks are overlain by The Rohkunborri Nappe fine- to medium-grained dark-greenish foliated and often banded amphibolites; no igneous parageneses or textures The Rohkunborri Nappe has a tectonostratigraphic are preserved. Above these, there is a characteristic kyan­ thickness of as much as 1000 metres in the Rohkunborri ite-bearing augen gneiss with metabasic lenses and locally area and c. 800 metres in the Njunis area. Both areas retrograded eclogites, in the area to the south of have a number of glacial cirques that provide three­ Gævdnjajavri. This unit is a good marker horizon that dimensional views of the internal parts of the nappe. can be mapped through the whole Altevatn-Dividalen More deformed parts of the Rohkunborri Nappe are area northwards to Måskanvarri (Fig. 3), near Trerik- exposed in the Kistefjell and Jerta areas. NORSK GEOLOOISK TIDSSKRIFT 74 (1994) Mafic dyke comp/ex, Rohkunborri Nappe, Troms 39

The Njunis succession appears to be jmbricated, be­ cause a repetition of parts of the internal tectonostratig­ raphy has been found. At least two major internal thrusts have been mapped (Fig. 6) within the Giim­ mariehppi area. Small-scale thrust faults are common and all kinematic indicators show top-to-the-east move­ ment. A late NW-dipping post-thrusting normal (Fig. 6) also occurs within the Njunis area.

Stratigraphy of the Rohkunborri Nappe

Although the Rohkunborri Nappe is made up of 65- 70% dykes, a sedimentary succession, the Njunis Group, can be recognized (Fig. 7), with an estimated thickness of > 25 00 metres. The lower part of the succession in the Rohkunborri and Njunis areas can be correlated. In the latter, the stratigraphy is more complex in the upper part, and repetitions due to folding and thrusting (see below) of the dyke complex complicate the Njunis stratigraphy. The Njunis area has a more varied strati­ graphy, containing more rock units in the upper part. screens that occur between multiple dykes can be thoroughly contact metamorphosed. Garnet-diop­ side-calcite scarns are frequently found near contacts to Fig. 4. Part of the western wall in the Giimmariehppi glacier cirque. Screens of dykes. Usual textures are irregular systems and nests sedimentary rockscan be seen in between areas dominatedby mafic rocks. Height consisting of numerous garnet crystalloblasts. Back vein­ of section is 350 m. ing is common in these contact metamorphosed areas, with veins of melted sediments intruding the dykes. Most The Rohkunborri Nappe is dominated is dominated of the rocks are rusty owing to impregnation by by mafic dykes, occurring in multiple sheeted complexes pyrrhotite and pyrite. Bedding surfaces dip steeply to the up to 100 metres wide (Fig. 4), or as single dykes, SSE in the Njunis area and cross-bedding and graded intruding a metasedimentary succession. The nappe oc­ bedding are found at a few localities in both areas, curs as a flat-lying klippen in the Rohkunborri area (Fig. indicating that the sediments are younging towards the 5), and in an open N-S trending synform in the Njunis southeast. The Njunis Group can be divided into four area. In the northern part of the Njunis area, the nappe formations: is refolded in a tight NE-SW trending synform. The central parts are particularly well preserved, and high­ (l) Høgskardet fo rmation: The lowermost formation of angle dyke-bedding relationships, chilled margins and the Njunis Group contains several horizons of dyke-in-dyke relationships can be observed (Fig. 6). The graphitic and psammitic schists, interbedded with dykes are deformed and amphibolitized towards the roof marble. lts lower boundary is a thrust contact to the and ftoor thrusts and in internal shear zones. Kali Nappes.

NW r�

5km

Fig. 5. Schematic profile through the Rohkunborri dyke complex. SNC = Seve Nappe Complex, MA = Middle Allochthon. Vertical scale is exaggerated. 40 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFT 74 (1994)

? NW SE

3

1?§:1 Rohkunborri Nappe l < < l Seve Nappe Complex l §l Sediment screens '''***'''''''''*'''''l Middle Allochthon Fault ' --- - - Dividal Group ,.- Lithological boundary way up

Fig. 6. Schematic profile through the central part of the Nj unis dyke complex with inset detailed drawings of selected sections. Thrust fa ults are shown as thick lines within the main drawing. Drawing l shows shallowly NW-dipping dykes and sub-vertical bedding within the sediment screens. Drawing 2 shows steep dykes and horizontal bedding in the west wall of Njunis. The angular unconfonnity between limes tones and overlying shales is marked with a dashed line. A thrust fa ult is shown as a thick line. Drawing 3 shows the same dyke-bedding relationships as in drawing l. A NW-dipping post-thrusting normal fault is found within this section.

(2) Bangfjellet fo rmation: The main outcrop is in the A series of rusty quartzites, as suhordinate concordant Bangfjellet area. The contacts are not exposed. This lenses within foliated amphiholites, occurs dose to the succession consists of yellow to grey dolomite several floor thrust of the Rohkunhorri Nappe, with an unclear hundred metres thick, interhedded with 2-8 cm thick relationship to the Njunis Group. No primary structures calc-silicate and scapolite layers. These layers make are found within these metasediments and the dyke-bed­ up more than 50% of the rock. ding relationships have heen rotated into concordance. (3) Sandjjellet fo rmation: A local angular unconformity (30°) hetween marhle and underlying units represents the lower houndary to the Sandfjellet formation in Mafic dykes of the Rohkunborri Nappe the Njunis area. The lower contact is not exposed in the Rohkunhorri area. Calcareous schists and im­ In the better-preserved parts of the nappe, medium- to pure marhles with calc-silicate hands form the major fine-grained grey dolerites with ophitic to suh-ophitic part of the succession in the Rohkunhorri area. textures, primary plagioclase phenocrysts and, locally, Marhle layers are often recrystallized to grossular, pyroxene are preserved. Early coarse-grained gahhroic diopside and calcite. A minor conglomerate horizon dykes are found in a few places and coarse gahhroic occurs in the lower part of the formation in the xenoliths are also found at a numher of localities in the Njunis area, containing a few granitoid pehhles in a dolerites. The width of the dykes follows a power law mudstone matrix. distrihution, with most dykes varying from 0.5 m up to (4) Giimmariehppi fo rmation: This unit is made up of a 3 m (Figs. 8, 9) and with a small number of thin dykes medium- to coarse-grained grey marhle, which (cm wide) and large dykes (up to 10 m wide). The dykes makes up a large part of the eastern part of the generally strike NNE and di p slightly towards the NW. Rohkunhorri massif. Calc-silicate and scapolite They cut the host metasediments at a high angle. Thin hands are common within marhle horizons. (cm wide) fine-grained dyke apophyses are found in parts NORSK GEOLOGISK TIDSSKRIFT 74 (I994) Mafic dyke complex, Rohkunborri Nappe, Troms 41

Grey homogeneous marble

Calcareous schist with subordinate layers of psammitic schist and marble

Rusty schist with WlCOnformity to marble

Calcareous schist Pelitic schist with subordinate marble andconglomerate scapolite schist ?

Dolomite with calc-silicate and scapolite layers

......

� Graphitic schist, psanunitic schist andmarble

;,.;; �J - Unclearrelationship to the rest of the sttatigraphy Quartzite

ROHKUNBORRI NJUNIS

Fig. 7. Schematic internal tectonostratigraphy of the Rohkunborri Nappe in the Njunis and Rohkunborri areas.

of the Njunis area, some extending as much as 2 m away (Fig. Ila), indicating deformation during the intrusive from the dyke. En echelon veins separated by bridge history. structures are found in areas where the is calc-silicate rich or consists of marble. Composite and fe /sic dykes Several dyke generations, based on cross-cutting rela­ tionships, are found, with younger dykes cutting older at Felsic dykes are rare within the dyke complexes and have angles of up to 40°, probably indicating dyke intrusion been found only at three localities in the Rohkunborri during active faulting. Late dykes have a width of 0.5- area and at two localities in the Njunis area. One of the 1.5 m. Single-sided chilling is commonly found, with Rohkunborri dykes starts as a composite mafic-felsic younger dykes intruding the central parts of older dykes. dyke, with a gradational change from the mafic to the Up to six single-sided chilled margins ( dyke-in-dyke in­ felsic components; it evolves into a granitoid dyke along trusions) have been found in the Rohkunborri area, strike and cuts the contact of the adjacent mafic dyke. comparable to multiple intrusions in sheeted dyke com­ The felsic dykes are medium grained and up to 2 m thick. plexes. The chilling direction is always towards the The main minerals are quartz, plagioclase (partly altered screens of metasedimentary rocks. to sericite) and K-feldspar (locally perthitic). Minor Two generations of sills, intruding along calc-silicate amounts of biotite, muscovite, chlorite, gamet and epi­ layers in marbles, are observed at one locality in the dote are also found. Titanite, zircon and opaques occur Giimmariehppi glacier cirque (Fig. lO); they are cut by as accessories. They can be followed for several hundreds later dykes. Small sidesteps (Walker 1987), l-2m long, of metres, clearly cutting the surrounding mafic dykes. A are observed at the lithological boundary between marble trondjemitic dyke occurs in the lower part of the Njunis and schist at a few localities in the Njunis area. succession, intruding the neighbouring mafic dykes at a Amphibolitization occurs dose to the roof and ftoor small angle. thrusts and in intemal shear zones. The amphibolites carry homblende and plagioclase with minor amounts of Pre- or syn-intrusion structures in the dyke complex quartz, zoisite and gamet. There are also a few examples of non-foliated dykes that are amphibolitized. One local­ Extension structures that can be related to deformation ity shows foliated dykes cut by later massive intrusions prior to or contemporary to dolerite intrusion can be 42 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFT 74 (1994)

Fig. 8. Detailed drawing of the lower part of the Giimmariehppi west wall showing the unconformity and the thrust fa ult. A marks the location of Fig. 9 and B marks the location of Fig. l O.

Fig. 9. Mafic dykes several metres thick with thinner dykes cross-cutting banded calc-silicate limestone at the inner part of the Giimmariehppi glacier cirque in the Njunis area, UTM 34WDB382272.

found at a few localities. These structures include pull­ orientations, but with the same or unclear age relation.­ apart - of competent calc-silicate layers in a less ships, might have intruded along complementary sets competent marble (Fig. llb) and small-scale faulting of of fractures related to normal faults in an extensional sedimentary beds (Fig. llc)_ environment. Most dykes occur in clusters with subparallel dykes Single zircon 207Pbj2°6Pb dating by direct evaporation. intruded along the margins of or in the central parts of (L. P. Gromet, pers_ comm_) of a felsic dyke in the earlier dykes. Dykes constitute up to nearly 100% of the Rohkunborri area and the felsic portion of a composite width of the clusters, with screens of metasedimentary maficjfelsic dyke at Njunis provide evidence of composite rocks between_ Sheeted complexes of dykes indicate that zircons containing a varied Precambrian inheritance with early dykes and their margins were planes of structural minimum ages ranging from � 780 to 1120 Ma (4 grains weakness offering easy paths for later intrusions. Later ana1ysed) and � 15 1O to 1885 Ma ( 2 grains analysed), dykes, which cut earlier dykes at angles of up to 45°, respectively. Several of the zircons displayed increasing might indicate tilting of crustal blocks during mag­ 207Pb/206Pb ages with progressive evaporation, suggestive matic activity, suggestive of intrusion in an active rift of core-overgrowth relationships. One zircon from the environment, with block-faulting. Dykes with different Rohkunborri sample displayed multiple early evapora- NORSK GEOLOGISK TIDSSKRIFT 74 (1994) Mafic dyke camp/ex, Rohkunborri Nappe, Troms 43

Fig. 10. cut by later dyke generations in the inner part of the Giimmariehppi glacier cirque, UTM 34WDB382272. The numbers indicate the order in which the dykes intruded: l. earl y si lis, 2. later cross-cutting dykes. The arrow marks the location of pull-apart-boudinage shown in Fig. Il b.

Fig. Ila. Amphibolitized dyke cut by later intrusion (x), indicating that parts of Fig. Ile. Small-scale extensional fa ulting in primary calc-silicate layer. Scale the dyke complex were amphibolitized prior to later intrusive events. Scale lO cm. Locality close to the summit area of Njunis, UTM 34WDB389271. 52 mm. UTM 34WDB388270.

tions of :::::: 600 Ma, which possibly represents the age of a magrnatic overgrowth formed on inherited zircon, and therefore the age of dyke crystallization. Although these data provide limited information on the age of the dykes, they provide very strong evidence for a Precambrian inheritance. The inherited zircons are interpreted to have originated by assimilation at deeper crustal levels, and indicate the presence at depth of Precambrian continental crust or its derivatives at the time of dyke emplacement. The Grenvillian and older ages for the inherited components favour a Baltoscan­ dian rather than an outboard source. Geochemically the dykes are tholeiites with MORB affinity and are interpreted to have formed by spreading­ related magrnatism, probably during the rift phase of the Fig. /lb. Pull-apart-boudinage of a more competent calc-silicate layer in lime­ stone. Width of section 0.5 m. Locality in the inner part of the Giimmariehppi Iapetus Ocean. The geochemistry of the dykes is treated glacier cirque, UTM 34WDB382272. elsewhere (Stølen, unpublished). 44 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFf 74 (1994)

Caledonian deformation of the dyke complex Dyke relationship A. - These dykes (Fig. 12a) cut bed­ ding at angles between 90° and 75°. lgneous textures and Dyke -bedding re/ationships commonly primary minerals are preserved and no folia­ Although the dyke complexes occur in low-strain zones, tion is developed. The dyke-bedding contact is planar they are influenced by the same deformation phases as the (Fig. 13) except for very thin dykes, where the contact is rock units below and above - mainly structures related irregular. The chilled margin may be up to 5 cm wide. to the 02 and 03 deformation phases. The deformation The contact-metamorphosed zone at the margins of the of the dyke complexes involves rotation, shearing and dykes varies from 2-4 cm up to several dm depending on folding. dyke thickness and number of dykes. This dyke-bedding The following section contains a description of this de­ relationship is found mainly in central parts of the dyke formation and an interpretation of its mechanism. There complex, but also in low-strain zones close to the mar­ are systematic relationships between dyke attitude and the gins of the complex. types of structure formed at the dyke margins (Fig. 12). Dyke relationship B. - In this case the angle between A dyke and bedding is between 75° and 30° (Figs. 12b, 12c). Folds have formed adjacent to the dykes and an axial plane (S3) is developed within dyke mar­ gins. Relict igneous textures can be found in the central parts of the dykes. Both symmetric and asymmetric folds are formed adjacent to dyke margins; symmetric folds are more common in the less deformed parts of the dyke complex and asymmetric folds occur in parts of the dyke complex with higher shear strain. An within dykes is observed at a few localities.

Dyke relationship C. - This relationship is found close to the roof and floor thrusts and near internal shear zones. The angle between dykes and bedding is smaller than 30° (Fig. 12d) and commonly the dykes and bedding are parallel to subparallel with a penetrative foliation devel­ oped both in the dyke and in the surrounding metasedi­ ments. Shear strain is more or less homogeneous throughout the dykes, except along the margins where a strain concentration can be found following the earlier c chilled margin (Fig. 14). Folds against dyke margins occur as sheared out isoclinal folds, with a mylonitic zone up to 5 cm wide close to the dyke margin often following the earlier contact-metamorphosed zone (Fig. 14). Boudinage involving both dykes and metasediments

D

Fig. 12. Examples of fold structures developed at the margins of dykes. Thick lines = dykes, thin lines = bedding surfaces. A: type A dyke-bedding relationship; Fig. 13. Bedding cut by mafic dyke at a high angle. The dyke-bedding contact is B: type B dyke-bedding relationship; C: evolved version of type B; D: type C planar. Scale 0.55 m. Locality on the southem side of Njunis, UTM dyke-bedding relationship. 32WDB379258. NORSK GEOLOGISK TIDSSKRIFT 74 (I994) Mafic dyke complex, Rohkunborri Nappe, Troms 45

INITIAL STAGE

Fig. 14. Folds developed at the margin of mafic dyke. A Mylonitic zone following the earlier contact-metamorphosed zone (marked with X). The dyke margin is also penetratively foliated, possibly along earlier chilled margin(m arked with Y). Scale 0.5 m. Locality SW part of Giimmariehppi glacier cirque, UTM 34WDB378259. is found in the lower part of the dyke complex in both the Rohkunborri and Njunis areas. The dykes are almost completely amphibolitized, igneous textures are rare and no primary minerals are found. STAGE 2

Structura/ evolution

In order to establish the structural evolution of the dyke complexes, an estimate of the original orientation (pre-deformational) of the dykes in relationship to the sedimentary bedding is necessary. Evidence from the best-preserved parts of the dyke complexes indicates that -- the initial dyke-bedding relationship was orthogonal to suborthogonal.

Rotation of dykes and bedding. - The rotation of the dykes and the bedding and subsequent folding of the latter involve at least two deformation phases (D2 and D3). During the first stage of deformation, the dykes acted as competent layers owing to the ductility contrast between the dykes and the surrounding metasediments. The initial rotation of the dykes was due to simple shear paraBel to the sedimentary bedding (Fig. 15) until D2 shear strain was unable to rotate the dykes and the dykes Fig. 15. Sketch of deformation relationships and formation of folds at dyke started acting as planes of weakness (Fig. 15), with most margins. Initial stage: inferred dyke-bedding relationship prior to deformation. Stage l: dykes rotate owing to simple shear parallel to sedimentary bedding. Stage of the strain localized along dyke margins. The following 2: dykes cannot be rotated further and start acting as shear planes. Folding of D3 deformation resulted in folding of the sedimentary bedding adjacent to dykes. Development of foliation in dyke margins. Folds form bedding adjacent to the dyke margins. The folding on both sides of dyke5. Dyke-bedding relationships suggest dykes were intruding along axial plane, but folds are formed after intrusion. started as symmetric folds, which were rotated into asymmetric folds with increasing shear strain. The above sequence of deformation is one explanation Reiz 1993); no such relationships have been observed of the dyke-bedding relationships observed in field. A within the Rohkunborri Nappe. different possibility is that the dykes intruded parallel to The sequence of deformation of dykes of bedding the axial planes of previously formed folds. If the folds within the Rohkunborri Nappe is in agreement with fold were formed prior to dyke intrusion, one would expect to formation at the margins of dykes in mafic dyke swarms see the same bed cut by the dykes on both limbs of the in the central (Krilll986) and northern (Gayeret al. 1978; fold in a single section normal to the fold axis ( Rice & Rice 1986) Scandinavian Caledonides. This statement 46 L. K. Stølen NORSK GEOLOGISK TIDSSKRIFT 74 (1994) does not, however, exclude the possibility that dykes dominated by calcareous sediments and dolomitic marble. cutting earlier folds exist in this part of the northern Sedimentary, extensional and magmatic structures are Norwegian Caledonides. common. The sedimentary rocks are supposed to repre­ sent the outermost part of the clastic wedge deposited during Neoproterozoic-earliest Palaeozoic time on the thinned outmost part of the Baltoscandian margin. Tectonostratigraphic correlations A complex structural evolution can be recognized Correlation of the Rohkunborri Nappe with other dyke­ within the dyke complexes invo1ving early block rotation intruded tectonic units in Troms (Figs. l, 2) as well as during intrusion, rotation of both dykes and bedding due the Norrbotten area in Sweden has been mentioned to simple shear deformation during thrusting and subse­ above. Other potential correlations should be considered. quent imbrication of the dyke complex. The most likely northward correlative of the Rohkun­ The Indre Troms dyke swarm, the Rohkunborri borri Nappe is the Corrovarre Nappe (Zwaan & van Nappe, is correlated with similar mafic intrusive events, Roermund 1990), although the latter is more psammite related to the opening of the lapetus Ocean, within the dominated than the Rohkunborri Nappe. Seve Nappe Complex in the Sarek, Kebnekaise and The host rocks of the mafic dyke swarms of the Tornetrask areas in northernmost Sweden. Rohkunborri Nappe show similarities with the Sarek­ tjåkkå Nappe (Svenningsen in press b), the Tsakkok Acknowledgements.- Financial support for fieldwork was provided by NGU, the Nappe (Kullerud et al. 1990) and the Vaivvancohkka Geological Survey of Norway and the Swedish Science Natura! Research Council (NFR) grant G-GU 1639-308. Statens Skoger, Troms Forvaltning, is thanked for Nappe (Kathol 1989) in Norrbotten. The Tsakkok permission to use a helicopter within the Dividal National Park. Abisko Nappe includes carbonate platform rocks and is thought Naturvetenskapliga Station is thanked for assistance during the fieldwork. to have originated at the outermost part of the Bal­ Fredrik Juel Theisen is thanked for help with the climbing during fieldwork and Ingvar Mattson provided able field assistance. L. P. Gromet, Brown University, toscandian margin; these rocks have suffered high-P is thanked for performing the age determinations on the dykes. David G. Gee and metamorphism during the Late Cambrian-Early Ordovi­ Per-Gunnar Andreasson are thanked for reading early versions of this paper. cian (Kullerud et al. 1990), an event that is not observed Constructive reviews by Peter Robinson, Brian Robins and Hugh Rice signifi­ cantly improved this paper. in the Rohkunborri Nappe. The of the screen rocks within the dyke com­ Manuscript received April 1993 plexes (see above), new geochemical (Stølen in prep) and isotopic data (see above) on the mafic dykes, together with tectonostratigraphic correlations with similar mafic References dyke complexes within the Seve Nappe Complex, sug­ gested a correlation with the Seve - suggested Andreasson, P. G. 1986: The Sarektjåkkå Nappe, Seve terranes of the northern Swedish Caledonides. Geo/ogiska Foreningens i Stockholm Forhand/ingar 108, earlier as a working hypothesis (Andreasson et al. 1988; 336-340. Stølen 1989) - rather than a correlation with the exotic Andreasson, P. G. 1987: Early evolution of the Late Proterozoic Baltoscandian Vaddas/Sulitjelma Terrane (Roberts 1988) equivalent to margin: inferences from rift magmatism. Geologiska Foreningens i Stockholm 12 Forhand/ingar 109, 378-379. Terrane of Stephens & Gee ( 1989) as abstracted Andreasson, P. G. & Gee, D. G. 1989: Bedrock geology and morphology of the earlier (Stølen 1991). Tarfala area, Kebnekaise Mts. Geografiska Annaler (71A), 235-239.

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